The present invention relates generally to wireless communications devices and, more particularly, to antennas used in such devices.
Many communications devices require antennas that are packaged within a small device or product. Common examples of such communications devices include portable communications products such as cellular handsets, personal digital assistants (PDAs), and wireless networking devices or data cards for personal computers (PCs). These devices often use a single antenna for both transmission and reception of wireless signals.
A conventional approach is to use a single port antenna for both transmit and receive functions. Because the local transmit signal is at a much higher power than the receive signals, a substantial amount of isolation between transmit and receive paths is needed, particularly because transmit and receive paths are connected at a common point at the antenna port. For time division duplexed architectures, the isolation is typically provided by a transmit/receive (TX/RX) select switch so that the antenna is only connected to the transmit circuitry during the transmit period, and only to the receive circuitry during the receive period. In the case of full duplex architectures, the isolation is obtained through use of a duplexer. In either case, because the transmit and receive frequency bands are slightly offset from each other, additional isolation is obtained by use of narrow band pass filters in particular in the receive circuitry.
An alternate approach is to use two separate antennas, one for transmit and one for receive, thereby relieving the isolation requirement of either the switch or duplexer because the transmit and receive paths are no longer connected at a common point. However, in general this is of limited utility for a handset or other portable wireless communication devices because the addition of a second antenna to the handset generally results in a two-antenna system where one antenna port is poorly isolated from the other due to electromagnetic coupling between the antennas and by coupling through a common ground structure. This coupling is problematic in handheld wireless devices for several reasons. First, at the desired frequencies of operation such as the cellular band (approximately 900 MHz), the size of a handset does not allow for antennas to be placed more than a fraction of a wavelength apart. Second, because consumer acceptance requires antennas to be embedded (or very low profile) such that the major portion of the antenna is provided by the phone chassis itself while the “antenna” may be better described as an exciter or a coupler-antenna, which transmits energy between the chassis and the antenna ports. Therefore, a two antenna approach may still in large part provide a common connection to a single antenna, i.e., the chassis. Furthermore, the operable bands of the antennas tend to overlap such that isolating the antennas by filtering (i.e., diplexing) is problematic. The bandwidth of a single antenna resonance is described by the antenna Q, and the number of poles characteristic of the resonators comprising the antenna system. In typical handsets, this is a two or 4-pole system, and does not have sufficient selectivity to isolate the receive and transmit band structure.
In applications where it is desirable to relax the isolation requirement of the switch, it is generally necessary to provide greater decoupling of the receive and transmit antennas. In accordance with one or more embodiments, a technique is provided utilizing a unique two-port antenna that may be embedded in a handset to achieve substantial isolation between ports thereby providing a means to realize the advantage of separate TX and RX ports. This method has the advantage that the requirement for a TX/RX switch or duplexer may be eliminated altogether or the performance requirements for these components may be relieved allowing for simpler or more cost-effective alternatives.